The multidrug resistance phenotype is dependent upon the expression of molecular pumps that are capable of expelling chemotherapeutic agents from their site of action in cancer cells. These molecular pumps include, for example, the P-glycoprotein (hereinafter: P-gp) pump, and the multidrug resistance-associated protein (hereinafter: MRP) pump, which are encoded by the MDR1 and MRP genes, respectively.
The discussion set forth below illustrates the problems faced by clinical investigators seeking to improve cancer treatments. A number of references have been included to describe the general state of the art. Inclusion of these references is not an admission that such references represent prior art with respect to the present invention.
Multidrug resistance (hereinafter "MDR") is largely dependent on the expression of one or the other or both of two different genes. These genes encode transmembrane energy-dependent molecular "pumps" that expel a wide variety of anticancer agents from their site of action in tumors (Grant et al., Cancer Res. 54: 357-361, 1994; Riordan and Ling, Pharmacol. Ther. 28: 51-75, 1985). The normal functions of these molecular pumps is not well defined, but both are expressed by hematopoietic cells and P-gp has been shown to have a causal role in immunofunctioning (Gupta et al., J. Clin. Immunol. 12: 451-458, 1992). The MRP is a molecular pump initially found to be involved in multidrug resistance in lung cancer, and then later found to be expressed in other cancer types, while P-gp is a molecular pump long known to be involved in producing multidrug resistance in many tumor types (Chin et al., Adv. Cancer Res. 60: 157-180, 1993; Cole et al., Science 258: 1650-1654, 1992; Grant et al., Cancer Res. 54: 357-361, 1994; Krishnamachary and Center, Cancer Res. 53: 3658-3661, 1993; Zaman et al., Cancer Res. 53: 1747-1750, 1993).
Clinical studies in which P-gp inhibitors were administered prior to chemotherapy showed that such competitive inhibitors could increase the response of the tumors to the anticancer agents without causing an equivalent increase in toxicity to normal tissues (Marie et al., Leukemia 7: 821-824, 1993; Miller et al., J. Clin. Oncol. 9: 17-24, 1991; Pastan and Gottesman, Annu. Rev. Med. 42: 277-286, 1991; Raderer and Scheithauer, Cancer 72: 3553-3563, 1993; Sonneveld et al., Lancet 340: 255-259, 1992). Oligos designed to block the expression of MRP or P-gp have several features which should make them more clinically effective than any of the existing competitive inhibitors of P-gp or to any comparable inhibitors for MRP.
First, most chemical inhibitors used clinically to combat multidrug resistance have serious side effects unrelated to their ability to inhibit P-gp. In contrast, the phosphorothioate oligo, OL(1)p53, has been found to be essentially devoid of any toxicity when administered to patients (Bayever et al., Antisense Res. Dev. 2: 109-110, 1992; Antisense Res. Dev., in press, 1994). Similarly, this and other phosphorothioates have been shown to be nontoxic to a variety of animal species, even when given at high doses (Cornish et al., Pharmacol. Com. 3: 239-247, 1993; Crooke, Ann. Rev. Pharm. Toxicol. 32: 329-376, 1992; Iversen, Anti-Cancer Drug Design 6: 531-538, 1991). These findings show that at least some types of oligo have no acute toxicity per se when given systemically to animals or patients.
Second, some oligos, including phosphorothioates, have been shown often to have an RNAse-H dependent mechanism of action (Crooke, Ann. Rev. Pharm. Toxicol. 32: 329-376, 1992). RNAse-H enzyme activity is often expressed in clonogenic cells, while little or no activity is found in differentiated (non-proliferative) cells (Papaphilis et al., Anticancer Res. 10: 1201-1212, 1990; Crooke, Ann. Rev. Pharm. Toxicol. 32: 329-376, 1992). Because of this, blocking MRP or P-gp synthesis with oligos (as opposed to blocking their function by competitive inhibitors) should be relatively more effective in proliferating than in non-proliferating cells. Most normal cells that express P-gp or MRP are non-proliferating. For example, gastrointestinal crypt cells (stem cells) do not express P-gp, whereas the endstage (non-proliferating) luminal cells do (Chin et al., Adv. Cancer Res. 60: 157-180, 1993). Furthermore, once MRP or P-gp synthesis is blocked, the remaining membrane-associated drug-efflux pump of the parent cell would then be divided between the two daughter cells, reducing the effective amount of the molecular pump in the proliferating tumor cell population by one-half for each population doubling.
In addition, several recent papers report that ODNs not only are capable of blocking the expression of particular genes in vitro, but also are able to produce this effect in vivo. Some groups have successfully inhibited HIV gene expression (including tax) in human cells in xenogeneic transplant models (Kitajima et al., J. Biol. Chem. 267:25881-25888, 1992). Others have targeted genes in cancer cells, including c-myc, c-Ha-ras, NF-kB, c-myb, c-kit and bcr-abl. In each of these instances involving the administration of ODNs to treat animals with xenogeneic human cancers, the transplanted malignant cells were found to regress (Agrawal et al., Proc. Natl. Acad. Sci. 86:7790-7794, 1989; Proc. Natl. Acad. Sci. 88:7595-7599, 1991; Biro et al., Proc. Natl. Acad. Sci. 90:654-658, 1993; Gray et al., Cancer Res. 53:577-580, 1993; Higgins et al., Proc. Natl. Acad. Sci. 90:9901-9905, 1993; Ratajczak et al., Proc. Natl. Acad. Sci. 89:11823-11827, 1992; Wickstrom et al., Cancer Res. 52:6741-6745, 1992).
Furthermore, the Food and Drug Administration has approved several phosphorothioate antisense oligonucleotides for systemic administration to patients and for ex vivo treatment of hematopoietic stem cell grafts. These approvals include the now-completed OL(1)p53 phase I clinical trials (both systemic and ex vivo administered) which targeted transcripts of the p53 gene in patients with acute myeloid leukemia (Bayever et al., Antisense Res. Develop. 2: 109-110, 1992; Karp and Broder, Cancer Res. 54: 653-665, 1994). Thus, antisense oligonucleotides have the pharmacologic properties necessary for use as drugs.
There are six reports claiming reduced drug resistance in cultured cell lines following treatment with oligos targeting MDR-1 mRNA. Three of these (Vasanthakumar & Ahmed, Cancer Com. 1: 225-232, 1989; Rivoltini et al., Int. J. Cancer 46: 727-732, 1990; Efferth & Volm, Oncology 50:303-308, 1993) are totally unconvincing because they used oligos directed against mouse MDR-1 to treat human cells; in the corresponding human MDR-1 sequence, the longest matching nucleotide sequence was only 6 bases long. The paper by Thierry et al. (Biochem. Biophys. Res. Comm. 190: 952-960, 1993) reports no oligo with a sequence which matches the human MDR-1 gene, but this problem is apparently due to typing errors (personal communication from A. Thierry). Thierry's 15-mer that gave 95% inhibition of MDR-1 expression did so only when encapsulated in liposomes; this was associated with a 4-fold increase in sensitivity of the tumor cells to doxorubicin (Thierry et al., Biochem. Biophys. Res. Comm. 190: 952-960, 1993); when administered without liposomes, inhibition of MDR1 expression was 40% of control values. Furthermore, the calculated melting temperature for Thierry's 15-mer is less than 28.degree. C., suggesting that at body temperature the amount of oligo bound is very low.
The most compelling papers in this group are by Jaroszewski et al. (Cancer Comm. 2: 287-294, 1990) and Corrias and Tonini (Anticancer Res. 12: 1431-1438, 1992). Both teams found inhibition with only one out of five oligos. Jaroszewski et al. (Cancer Comm. 2: 287-294, 1990) describe one phosphorothioate (which is being designated "Cohen(1)mdr" herein) that gave 25% reduction in P-gp expression at 15 .mu.M and 33% reduction at 30 .mu.M when incubated with MCF-7/ADR breast cancer cells for 5 days. This reduction in P-gp expression was associated with a small increase in the doxorubicin sensitivity of the cells (20% increase in cell death when 10 .mu.M of the oligo was used. Corrias & Tonini (Anticancer Res. 12: 1431-1438, 1992) report a phosphodiester oligo that gave only a slight reduction in P-gp (data not shown herein) at 30 .mu.M when incubated with doxorubicin-resistant colon adenocarcinoma cells for 36 hours. The reduction in P-gp expression was associated with a significant increase in the in vitro sensitivity of the cells to the cytotoxic effects of doxorubicin (80% and 53% dose reductions in IC.sub.50, respectively; the IC.sub.50 being the inhibitory concentration of a chemotherapeutic agent (e.g., doxorubicin) which causes a 50% inhibition in cellular proliferation).
It is, therefore, a principal object of the present invention to provide MDR-oligos or MRP-oligos that target the genes encoding P-gp or MRP, respectively, or their RNA transcripts, in order to specifically and effectively sensitize clonogenic multidrug-resistant tumor cells to chemotherapeutic agents. It is another object of the present invention to provide oligos which will sensitize tumor cells much more efficiently than they do normal cells which express these same molecular pumps. As the foregoing discussion highlights, oligonucleotides effective for these purposes heretofore have been unavailable.
There is growing evidence that certain protein kinases, such as protein kinase A (PKA), and protein kinase C (PKC) in particular, are involved in the activation of the forms of drug resistance which depend on the expression of molecules producing multidrug resistance, such as, for example, P-glycoprotein, MRP, pi-class glutathione S-transferase, gamma-glutamylcysteine (Gekeler et al., Biochem. Biophys. Res. Comm. 205: 119, 1995; Grunicke et al., Ann. Hematol. 69 (Suppl 1): S1-6, 1994; Gupta et al., Cancer Lett. 76: 139, 1994), or the transmembrane pump capable of expelling glutathione conjugates from their site of action in cells (such as the GS-X pump (Ishikawa et al., J. Biol. Chem. 269: 29085, 1994).
These second-messenger pathways typically appear to be more active in drug resistant cancer cells compared to their drug sensitive counterparts. These pathways promote the expression of various drug resistance phenotypes by causing the up-regulation of a very small number of specific transcriptional regulators, including AP-1, that presumably control the activation of molecules involved in producing drug resistance in cancer cells (Grunicke et al., Ann. Hematol. 69(Suppl 1): S1-6, 1994). For example, activation of the ras oncogene in malignant cells is one of the ways that PKC and, in turn, multidrug resistance, can be up-regulated in tumor cells. Hence, the realization that what has been considered to be multiple discrete mechanisms for drug resistance share common activation pathways opens a new set of possibilities for broad spectrum therapeutic interventions.
Thus, if inhibitors of these second messenger pathways could be found, they should be of use for treating a wider variety of multidrug resistance phenotypes in cancer than agents designed to inhibit the function or expression of a single molecular species involved in drug resistance (Grunicke et al., Ann. Hematol. 69(Suppl 1): S1-6, 1994; Christen et al., Cancer Metastasis Rev. 13: 175, 1994). In addition, inhibitors of particular PKC isoenzymes, or of PKA, or molecular regulators up- or down-stream of these enzymes, should find a variety of other applications where these protein kinases are known to play an important role, including the treatment of viral infections, AIDS, Alzheimer's Disease, and conditions where immunosuppression is important such as in autoimmune diseases, transplantation-related reactions and inflammatory reactions.
Stein et al. (Biochemistry 32: 4855, 1993) discovered that both a 15-mer phosphodiester homopolymer of thymidine and a 28-mer phosphorothioate homopolymer of cytidine could inhibit the B1 isoenzyme of PKC, with the result that pinocytosis and cellular uptake of macromolecules was inhibited. For the 28-mer phosphorothioate, the IC.sub.50 for directly inhibiting purified PKC-.beta.1 activity was 1 .mu.M; complete suppression required nearly 40 .mu.M.
Conrad et al (J. Biol. Chem. 269: 32051, 1994) have shown that certain RNA aptamers can inhibit the .beta.II isoenzyme of PKC. These RNA aptamers were selected from a pool of RNA molecules that contained a 120-nucleotide randomized region. PKC-.beta.I is an alternatively-spliced variant of PKC-.beta.II.
Schuttze et al (J. Mol. Biol. 235: 1532, 1994) have analyzed in detail the three-dimensional solution structure of the thrombin-binding DNA aptamer d(GGTTGGTGTGGTTGG). This aptamer binds to thrombin and inhibits its activity in the chain of reactions that lead to blood clotting. The authors conclude that "knowledge of the three-dimensional structure of this thrombin aptamer may be relevant for the design of improved thrombin-inhibiting anti-coagulants with similar structural motifs."
Using the human KM12L4a colon cancer cell line, Gravitt et al. (Biochem. Pharmacol. 48: 375, 1994) discovered that the agent thymeleatoxin (which stimulates the phorbol ester-responsive PKC isoenzymes -.alpha., -.beta.I, -.beta.II and -gamma) induces multidrug resistance. Since this cell line expresses only the PKC-.alpha. isoenzyme, it is clear that PKC-.alpha. lies in a second messenger pathway that can up-regulate multidrug resistance.
Fan et al (Anticancer Res. 12: 661, 1992) presented data showing that the expression of rat brain PKC-.beta.I confers a multidrug resistance phenotype on rat fibroblasts.
Thus, it is another object of the present invention to provide oligonucleotides that inhibit various MDR phenotypes in cancer cells by exerting an aptameric effect.